Active tuned networks using operational amplifiers may be characterized by three variables: center frequency, bandwidth and voltage gain. Center frequency may be controlled by the inductance of a network although, at lower frequencies, lumped inductances are either not readily available or are too large to be used in conjunction with electronic circuits. At higher frequencies, lumped inductances cannot be easily integrated either monolithically or in hybrid form into an integrated circuit. Consequently, it is necessary to synthesize an equivalent network inductance with resistive capacitive parameters to provide control of center frequency at low frequencies and in integrated circuit networks.
Analysis of presently available tuned networks reveals that the passive parameters controlling center frequency may also affect bandwidth and voltage gain. Changing the value of one parameter to adjust a single variable inevitably affects other variables. For example, in a Wien bridge used as a bandpass filter adjustment of frequency affects both bandwidth and gain. Additionally, instability due to temperature variations, inaccurate component values and changing component tolerances affect all network variables thereby complicating tuning.
A conventional approach toward providing some degree of independence between control of bandpass filter network parameters uses a resistive-capacitive pair in conjunction with an active device to synthesize a variable tuning inductor. A second resistive-capacitive pair coupled across the output terminal of the active device provides center frequency tuning adjustment independently of bandwidth and voltage gain adjustments. Voltage gain, however, changes slightly with adjustment of bandwidth. For this approach, it is necessary to match the resistive-capacitive pairs to synthesize a nearly ideal inductor and thereby heighten the degree of independence between adjustable circuit variables. The disadvantages of this type of network arise from its requirement for matched resistive-capacitive pairs, a requirement which increases both the number of component parts in the circuit and the cost of the circuit because of the greater expense in obtaining matched components. In an active bandpass filter network, a matched resistive-capacitive pair is usually more expensive than the active device alone.